Deborah A. Clark of the University of Missouri-St. Louis and her colleagues measured the annual growth of six species of trees in an old-growth rain forest in La Selva, Costa Rica, between 1984 and 2000. The researchers also used data from global climate monitoring stations to calculate CO2 emissions from tropical lands over the same time period. Tree growth and the amount of carbon dioxide exchange both varied greatly over the 16-year period, and both were correlated with temperature. In addition, during the warmest years--particularly the record-breaking 1997-1998 El Nino episode--the rain-forest trees experienced the least growth and expelled the most carbon dioxide, the scientists report. They conclude that the carbon balance of the La Selva rain forest is remarkably sensitive to increasing temperatures. Tropical rain forests could thus potentially induce a large positive feedback for global CO2 atmospheric accumulation. Note the authors: "Such a feedback in future years would accelerate global warming."